Managing Global Names in a Distributed System

In a distributed database system, each database should have a unique global database name. Global database names uniquely identify a database in the system. A primary administration task in a distributed system is managing the creation and alteration of global database names.

Understanding How Global Database Names Are Formed

A global database name is formed from two components: a database name and a domain. The database name and the domain name are determined by the following initialization parameters at database creation:

Component

Parameter

Requirements

Example

Database name

DB_NAME

Must be eight characters or less.

sales

Domain containing the database

DB_DOMAIN

Must follow standard Internet conventions. Levels in domain names must be separated by dots and the order of domain names is from leaf to root, left to right.

us.acme.com

These are examples of valid global database names:

DB_NAME

DB_DOMAIN

Global Database Name

sales

au.oracle.com

sales.au.oracle.com

sales

us.oracle.com

sales.us.oracle.com

mktg

us.oracle.com

mktg.us.oracle.com

payroll

nonprofit.org

payroll.nonprofit.org

The DB_DOMAIN initialization parameter is only important at database creation time when it is used, together with the DB_NAME parameter, to form the database's global name. At this point, the database's global name is stored in the data dictionary. You must change the global name using an ALTER DATABASE statement, not by altering the DB_DOMAIN parameter in the initialization parameter file. It is good practice, however, to change the DB_DOMAIN parameter to reflect the change in the domain name before the next database startup.

Determining Whether Global Naming Is Enforced

The name that you give to a link on the local database depends on whether the remote database that you want to access enforces global naming. If the remote database enforces global naming, then you must use the remote database's global database name as the name of the link. For example, if you are connected to the local hq server and want to create a link to the remote mfg database, and mfg enforces global naming, then you must use mfg's global database name as the link name.

You can also use service names as part of the database link name. For example, if you use the service names sn1 and sn2 to connect to database hq.acme.com, and hq enforces global naming, then you can create the following link names to hq:

To determine whether global naming on a database is enforced on a database, either examine the database's initialization parameter file or query the V$PARAMETER view. For example, to see whether global naming is enforced on mfg, you could start a session on mfg and then create and execute the following globalnames.sql script (sample output included):

Changing the Domain in a Global Database Name

Use the ALTER DATABASE statement to change the domain in a database's global name. Note that after the database is created, changing the initialization parameter DB_DOMAIN has no effect on the global database name or on the resolution of database link names.

The following example shows the syntax for the renaming statement, where database is a database name and domain is the network domain:

ALTER DATABASE RENAME GLOBAL_NAME TOdatabase.domain;

Use the following procedure to change the domain in a global database name:

Changing a Global Database Name: Scenario

In this scenario, you change the domain part of the global database name of the local database. You also create database links using partially-specified global names to test how Oracle resolves the names. You discover that Oracle resolves the partial names using the domain part of the current global database name of the local database, not the value for the initialization parameter DB_DOMAIN.

You connect to SALES.US.ACME.COM and query the GLOBAL_NAME data dictionary view to determine the database's current global name:

This result indicates that the domain part of the global database name of the local database is us.acme.com. Oracle uses this domain in resolving partial database link names when the database link is created.

Because you have received word that the sales database will move to Japan, you rename the sales database to sales.jp.acme.com:

This result indicates that the value of the DB_DOMAIN initialization parameter is independent of the ALTER DATABASE RENAME GLOBAL_NAME statement. The ALTER DATABASE statement determines the domain of the global database name, not the DB_DOMAIN initialization parameter (although it is good practice to alter DB_DOMAIN to reflect the new domain name).

You create another database link to database supply, and then query USER_DB_LINKS to see how Oracle resolves the domain part of supply's global database name:

This result indicates that Oracle resolves the partially specified link name by using the domain jp.acme.com. This domain is used when the link is created because it is the domain part of the global database name of the local database. Oracle does not use the DB_DOMAIN initialization parameter setting when resolving the partial link name.

You then receive word that your previous information was faulty: sales will be in the ASIA.JP.ACME.COM domain, not the JP.ACME.COM domain. Consequently, you rename the global database name as follows:

Obtaining Privileges Necessary for Creating Database Links

A database link is a pointer in the local database that allows you to access objects on a remote database. To create a private database link, you must have been granted the proper privileges. The following table illustrates which privileges are required on which database for which type of link:

Privilege

Database

Required For

CREATE DATABASE LINK

Local

Creation of a private database link.

CREATE PUBLIC DATABASE LINK

Local

Creation of a public database link.

CREATE SESSION

Remote

Creation of any type of database link.

To see which privileges you currently have available, query ROLE_SYS_PRIVS. For example, you could create and execute the following privs.sql script (sample output included):

A private link called link_1 to the database with service name us_supply. The link uses the userid/password of the current user to log onto the remote database.

Note: The current user may not be the same as the connected user, and must be a global user on both databases involved in the link (see "Users of Database Links"). Current user links are part of the Oracle Advanced Security option.

Creating Public Database Links

To create a public database link, use the keyword PUBLIC (where link_name is the global database name or an arbitrary link name):

CREATE PUBLIC DATABASE LINK link_name ...;

Following are examples of public database links:

This SQL Statement...

Creates...

CREATE PUBLIC DATABASE LINK supply.us.acme.com;

A public link to the remote supply database. The link uses the userid/password of the connected user. So if scott (identified by tiger) uses the link in a query, the link establishes a connection to the remote database as scott/tiger.

CREATE PUBLIC DATABASE LINK pu_link CONNECT TO
CURRENT_USER USING 'supply';

A public link called pu_link to the database with service name supply. The link uses the userid/password of the current user to log onto the remote database.

Note: The current user may not be the same as the connected user, and must be a global user on both databases involved in the link (see "Users of Database Links").

Creating Global Database Links

Specifying Link Users

A database link defines a communication path from one database to another. When an application uses a database link to access a remote database, Oracle establishes a database session in the remote database on behalf of the local application request.

When you create a private or public database link, you can determine which schema on the remote database the link will establish connections to by creating fixed user, current user, and connected user database links.

Creating Fixed User Database Links

To create a fixed user database link, you embed the credentials (in this case, a username and password) required to access the remote database in the definition of the link:

A private fixed user link called foo to the database with service name finance. The link connects to the remote database with the userid/password jane/doe.

When an application uses a fixed user database link, the local server always establishes a connection to a fixed remote schema in the remote database. The local server also sends the fixed user's credentials across the network when an application uses the link to access the remote database.

Creating Connected User and Current User Database Links

Connected user and current user database links do not include credentials in the definition of the link. The credentials used to connect to the remote database can change depending on the user that references the database link and the operation performed by the application.

Note:

For many distributed applications, you do not want a user to have privileges in a remote database. One simple way to achieve this result is to create a procedure that contains a fixed user or current user database link within it. In this way, the user accessing the procedure temporarily assumes someone else's privileges.

For an extended conceptual discussion of the distinction between connected users and current users, see "Users of Database Links".

Creating a Connected User Database Link

To create a connected user database link, omit the CONNECT TO clause. The following syntax creates a connected user database link, where dblink is the name of the link and net_service_name is an optional connect string:

In some situations, you may want to have several database links of the same type (for example, public) that point to the same remote database, yet establish connections to the remote database using different communication pathways. Some cases in which this strategy is useful are:

A remote database is part of an Oracle Real Application Clusters configuration, so you define several public database links at your local node so that connections can be established to specific instances of the remote database.

Some clients connect to the Oracle server using TCP/IP while others use DECNET.

To facilitate such functionality, Oracle allows you to create a database link with an optional service name in the database link name. When creating a database link, a service name is specified as the trailing portion of the database link name, separated by an @ sign, as in @sales. This string is called a connection qualifier.

For example, assume that remote database hq.acme.com is managed in a Oracle Real Application Clusters environment. The hq database has two instances named hq_1 and hq_2. The local database can contain the following public database links to define pathways to the remote instances of the hq database:

Notice in the first two examples that a service name is simply a part of the database link name. The text of the service name does not necessarily indicate how a connection is to be established; this information is specified in the service name of the USING clause. Also notice that in the third example, a service name is not specified as part of the link name. In this case, just as when a service name is specified as part of the link name, the instance is determined by the USING string.

To use a service name to specify a particular instance, include the service name at the end of the global object name:

SELECT * FROM scott.emp@hq.acme.com@hq_1

Note that in this example, there are two @ symbols.

Creating Shared Database Links

Every application that references a remote server using a standard database link establishes a connection between the local database and the remote database. Many users running applications simultaneously can cause a high number of connections between the local and remote databases.

Shared database links enable you to limit the number of network connections required between the local server and the remote server.

Determining Whether to Use Shared Database Links

Look carefully at your application and shared server configuration to determine whether to use shared links. A simple guideline is to use shared database links when the number of users accessing a database link is expected to be much larger than the number of server processes in the local database.

The following table illustrates three possible configurations involving database links:

Link Type

Server Mode

Consequences

Non-Shared

Dedicated/shared server

If your application uses a standard public database link, and 100 users simultaneously require a connection, then 100 direct network connections to the remote database are required.

Shared

shared server

If 10 shared server processes exist in the local shared server mode database, then 100 users that use the same database link require 10 or fewer network connections to the remote server. Each local shared server process may only need one connection to the remote server.

Shared

Dedicated

If 10 clients connect to a local dedicated server, and each client has 10 sessions on the same connection (thus establishing 100 sessions overall), and each session references the same remote database, then only 10 connections are needed. With a non-shared database link, 100 connections are needed.

Shared database links are not useful in all situations. Assume that only one user accesses the remote server. If this user defines a shared database link and 10 shared server processes exist in the local database, then this user can require up to 10 network connections to the remote server. Because the user can use each shared server process, each process can establish a connection to the remote server.

Clearly, a non-shared database link is preferable in this situation because it requires only one network connection. Shared database links lead to more network connections in single-user scenarios, so use shared links only when many users need to use the same link. Typically, shared links are used for public database links, but can also be used for private database links when many clients access the same local schema (and therefore the same private database link).

Creating Shared Database Links

To create a shared database link, use the keyword SHARED in the CREATE DATABASE LINK statement:

Whenever you use the keyword SHARED, the clause AUTHENTICATED BY is required. The schema specified in the AUTHENTICATED BY clause is only used for security reasons and can be considered a dummy schema. It is not affected when using shared database links, nor does it affect the users of the shared database link. The AUTHENTICATED BY clause is required to prevent unauthorized clients from masquerading as a database link user and gaining access to privileged information.

Configuring Shared Database Links

Creating Shared Links to Dedicated Servers

In the configuration illustrated in Figure 29-1, a shared server process in the local server owns a dedicated remote server process. The advantage is that a direct network transport exists between the local shared server and the remote dedicated server. A disadvantage is that extra back-end server processes are needed.

Note:

The remote server can either be a shared server or dedicated server. There is a dedicated connection between the local and remote servers. When the remote server is a shared server, you can force a dedicated server connection by using the (SERVER=DEDICATED) clause in the definition of the service name.

Figure 29-1 A Shared Database Link to Dedicated Server Processes

Creating Shared Links to Shared Servers

The configuration illustrated in Figure 29-2 uses shared server processes on the remote server. This configuration eliminates the need for more dedicated servers, but requires the connection to go through the dispatcher on the remote server. Note that both the local and the remote server must be configured as shared servers.

Managing Database Links

Closing Database Links

If you access a database link in a session, then the link remains open until you close the session. A link is open in the sense that a process is active on each of the remote databases accessed through the link. This situation has the following consequences:

If 20 users open sessions and access the same public link in a local database, then 20 database link connections are open.

If 20 users open sessions and each user accesses a private link, then 20 database link connections are open.

If one user starts a session and accesses 20 different links, then 20 database link connections are open.

After you close a session, the links that were active in the session are automatically closed. You may have occasion to close the link manually. For example, close links when:

The network connection established by a link is used infrequently in an application.

The user session must be terminated.

If you want to close a link, issue the following statement, where linkname refers to the name of the link:

ALTER SESSION CLOSE DATABASE LINK linkname;

Note that this statement only closes the links that are active in your current session.

Dropping Database Links

You can drop a database link just as you can drop a table or view. If the link is private, then it must be in your schema. If the link is public, then you must have the DROP PUBLIC DATABASE LINK system privilege.

The statement syntax is as follows, where dblink is the name of the link:

DROP [PUBLIC] DATABASE LINK dblink;

Procedure for Dropping a Private Database Link

Connect to the local database using SQL*Plus. For example, enter:

CONNECT scott/tiger@local_db

Query USER_DB_LINKS to view the links that you own. For example, enter:

Drop the desired link using the DROP PUBLIC DATABASE LINK statement. For example, enter:

DROP PUBLIC DATABASE LINK sales.us.oracle.com;

Limiting the Number of Active Database Link Connections

You can limit the number of connections from a user process to remote databases using the static initialization parameter OPEN_LINKS. This parameter controls the number of remote connections that a single user session can use concurrently in distributed transactions.

Note the following considerations for setting this parameter:

The value should be greater than or equal to the number of databases referred to in a single SQL statement that references multiple databases.

Increase the value if several distributed databases are accessed over time. Thus, if you regularly access three database, set OPEN_LINKS to 3 or greater.

The default value for OPEN_LINKS is 4. If OPEN_LINKS is set to 0, then no distributed transactions are allowed.

Viewing Information About Database Links

The data dictionary of each database stores the definitions of all the database links in the database. You can use data dictionary tables and views to gain information about the links. This section contains the following topics:

Authorization for Viewing Password Information

Only USER_DB_LINKS contains a column for password information. However, if you are an administrative user (SYS or users who connect AS SYSDBA), then you can view passwords for all links in the database by querying the LINK$ table. If you are not an administrative user, you can be authorized to query the LINK$ table by one of the following methods:

Viewing Authentication Passwords

It is possible to view AUTHENTICATED BY ... IDENTIFIED BY ... usernames and passwords for all links in the database by querying the LINK$ table. You can create and run the following script in SQL*Plus to obtain password information (sample output included):

Determining Which Link Connections Are Open

You may find it useful to determine which database link connections are currently open in your session. Note that if you connect as SYSDBA, you cannot query a view to determine all the links open for all sessions; you can only access the link information in the session within which you are working.

The following views show the database link connections that are currently open in your current session:

View

Purpose

V$DBLINK

Lists all open database links in your session, that is, all database links with the IN_TRANSACTION column set to YES.

GV$DBLINK

Lists all open database links in your session along with their corresponding instances. This view is useful in an Oracle Real Application Clusters configuration.

These data dictionary views contain the same basic information about database links, with one exception:

Column

Which Views?

Description

DB_LINK

All

The name of the database link.

OWNER_ID

All

The owner of the database link.

LOGGED_ON

All

Whether the database link is currently logged on.

HETEROGENEOUS

All

Whether the database link is homogeneous (NO) or heterogeneous (YES).

PROTOCOL

All

The communication protocol for the database link.

OPEN_CURSORS

All

Whether cursors are open for the database link.

IN_TRANSACTION

All

Whether the database link is accessed in a transaction that has not yet been committed or rolled back.

UPDATE_SENT

All

Whether there was an update on the database link.

COMMIT_POINT_STRENGTH

All

The commit point strength of the transactions using the database link.

INST_ID

GV$DBLINK only

The instance from which the view information was obtained.

For example, you can create and execute the script below to determine which links are open (sample output included):

Creating Location Transparency

After you have configured the necessary database links, you can use various tools to hide the distributed nature of the database system from users. In other words, users can access remote objects as if they were local objects. The following sections explain how to hide distributed functionality from users:

Using Views to Create Location Transparency

Local views can provide location transparency for local and remote tables in a distributed database system.

For example, assume that table emp is stored in a local database and table dept is stored in a remote database. To make these tables transparent to users of the system, you can create a view in the local database that joins local and remote data:

Figure 29-3 Views and Location Transparency

When users access this view, they do not need to know where the data is physically stored, or if data from more than one table is being accessed. Thus, it is easier for them to get required information. For example, the following query provides data from both the local and remote database table:

SELECT * FROM company;

The owner of the local view can grant only those object privileges on the local view that have been granted by the remote user. (The remote user is implied by the type of database link). This is similar to privilege management for views that reference local data.

Using Synonyms to Create Location Transparency

Synonyms are useful in both distributed and nondistributed environments because they hide the identity of the underlying object, including its location in a distributed database system. If you must rename or move the underlying object, you only need to redefine the synonym; applications based on the synonym continue to function normally. Synonyms also simplify SQL statements for users in a distributed database system.

Creating Synonyms

You can create synonyms for the following:

Tables

Types

Views

Materialized views

Sequences

Procedures

Functions

Packages

All synonyms are schema objects that are stored in the data dictionary of the database in which they are created. To simplify remote table access through database links, a synonym can allow single-word access to remote data, hiding the specific object name and the location from users of the synonym.

PUBLIC is a keyword specifying that this synonym is available to all users. Omitting this parameter makes a synonym private, and usable only by the creator. Public synonyms can be created only by a user with CREATE PUBLIC SYNONYM system privilege.

synonym_name specifies the alternate object name to be referenced by users and applications.

schema specifies the schema of the object specified in object_name. Omitting this parameter uses the creator's schema as the schema of the object.

database_link_name specifies the database link identifying the remote database and schema in which the object specified in object_name is located.

A synonym must be a uniquely named object for its schema. If a schema contains a schema object and a public synonym exists with the same name, then Oracle always finds the schema object when the user that owns the schema references that name.

Example: Creating a Public Synonym

Assume that in every database in a distributed database system, a public synonym is defined for the scott.emp table stored in the hq database:

CREATE PUBLIC SYNONYM emp FOR scott.emp@hq.acme.com;

You can design an employee management application without regard to where the application is used because the location of the table scott.emp@hq.acme.com is hidden by the public synonyms. SQL statements in the application access the table by referencing the public synonym emp.

Furthermore, if you move the emp table from the hq database to the hr database, then you only need to change the public synonyms on the nodes of the system. The employee management application continues to function properly on all nodes.

Managing Privileges and Synonyms

A synonym is a reference to an actual object. A user who has access to a synonym for a particular schema object must also have privileges on the underlying schema object itself. For example, if the user attempts to access a synonym but does not have privileges on the table it identifies, an error occurs indicating that the table or view does not exist.

Assume scott creates local synonym emp as an alias for remote object scott.emp@sales.acme.com. scottcannot grant object privileges on the synonym to another local user. scott cannot grant local privileges for the synonym because this operation amounts to granting privileges for the remote emp table on the sales database, which is not allowed. This behavior is different from privilege management for synonyms that are aliases for local tables or views.

Therefore, you cannot manage local privileges when synonyms are used for location transparency. Security for the base object is controlled entirely at the remote node. For example, user admin cannot grant object privileges for the EMP_SYN synonym.

Unlike a database link referenced in a view or procedure definition, a database link referenced in a synonym is resolved by first looking for a private link owned by the schema in effect at the time the reference to the synonym is parsed. Therefore, to ensure the desired object resolution, it is especially important to specify the underlying object's schema in the definition of a synonym.

Using Procedures to Create Location Transparency

PL/SQL program units called procedures can provide location transparency. You have these options:

When a user or application calls the fire_emp procedure, it is not apparent that a remote table is being modified.

A second layer of location transparency is possible when the statements in a procedure indirectly reference remote data using local procedures, views, or synonyms. For example, the following statement defines a local synonym:

CREATE SYNONYM emp FOR emp@hq.acme.com;

Given this synonym, you can create the fire_emp procedure using the following statement:

If you rename or move the table emp@hq, then you only need to modify the local synonym that references the table. None of the procedures and applications that call the procedure require modification.

Using Local Procedures to Call Remote Procedures

You can use a local procedure to call a remote procedure. The remote procedure can then execute the required DML. For example, assume that scott connects to local_db and creates the following procedure:

A local user on supply can use this synonym to execute the procedure on sales.

Managing Procedures and Privileges

Assume a local procedure includes a statement that references a remote table or view. The owner of the local procedure can grant the execute privilege to any user, thereby giving that user the ability to execute the procedure and, indirectly, access remote data.

In general, procedures aid in security. Privileges for objects referenced within a procedure do not need to be explicitly granted to the calling users.

Managing Statement Transparency

Queries including joins, aggregates, subqueries, and SELECT ... FOR UPDATE can reference any number of local and remote tables and views. For example, the following query joins information from two remote tables:

UPDATE, INSERT, DELETE, and LOCK TABLE statements can reference both local and remote tables. No programming is necessary to update remote data. For example, the following statement inserts new rows into the remote table emp in the scott.sales schema by selecting rows from the emp table in the jward schema in the local database:

In a distributed database system, Oracle always evaluates environmentally-dependent SQL functions such as SYSDATE, USER, UID, and USERENV with respect to the local server, no matter where the statement (or portion of a statement) executes.

Note:

Oracle supports the USERENV function for queries only.

A number of performance restrictions relate to access of remote objects:

There is a restriction in Oracle's implementation of distributed read consistency that can cause one node to be in the past with respect to another node. In accordance with read consistency, a query may end up retrieving consistent, but out-of-date data. See "Managing Read Consistency" to learn how to manage this problem.

Managing a Distributed Database: Scenarios

This section gives examples of various types of statements involving management of database links:

Consequences:

Any user connected to the local database can use the sales.division3.acme.com database link to connect to the remote database. Each user connects to the schema scott in the remote database.

To access the table emp table in scott's remote schema, a user can issue the following SQL query:

SELECT * FROM emp@sales.division3.acme.com;

Note that each application or user session creates a separate connection to the common account on the server. The connection to the remote database remains open for the duration of the application or user session.

Creating a Public Fixed User Shared Database Link

The following example connects to the local database as dana and creates a public link to the sales database (using its net service name sldb). The link allows a connection to the remote database as scott and authenticates this user as scott:

Consequences:

Any user connected to the local shared server can use this database link to connect to the remote sales database through a shared server process. The user can then query tables in the scott schema.

In the above example, each local shared server can establish one connection to the remote server. Whenever a local shared server process needs to access the remote server through the sales.division3.acme.com database link, the local shared server process reuses established network connections.

Creating a Public Connected User Database Link

The following example connects to the local database as larry and creates a public link to the database with the net service name sldb:

Consequences:

Any user connected to the local database can use the redwood database link. The connected user in the local database who uses the database link determines the remote schema.

If scott is the connected user and uses the database link, then the database link connects to the remote schema scott. If fox is the connected user and uses the database link, then the database link connects to remote schema fox.

The following statement fails for local user fox in the local database when the remote schema fox cannot resolve the emp schema object. That is, if the fox schema in the sales.division3.acme.com does not have emp as a table, view, or (public) synonym, an error will be returned.

CONNECT fox/mulder@local
SELECT * FROM emp@redwood;

Creating a Public Connected User Shared Database Link

The following example connects to the local database as neil and creates a shared, public link to the sales database (using its net service name sldb). The user is authenticated by the userid/password of crazy/horse. The following statement creates a public, connected user, shared database link:

Consequences:

Each user connected to the local server can use this shared database link to connect to the remote database and query the tables in the corresponding remote schema.

Each local, shared server process establishes one connection to the remote server. Whenever a local server process needs to access the remote server through the sales.division3.acme.com database link, the local process reuses established network connections, even if the connected user is a different user.

If this database link is used frequently, eventually every shared server in the local database will have a remote connection. At this point, no more physical connections are needed to the remote server, even if new users use this shared database link.

Creating a Public Current User Database Link

The following example connects to the local database as the connected user and creates a public link to the sales database (using its net service name sldb). The following statement creates a public current user database link:

When ford executes the procedure scott.fire_emp, the procedure runs under scott's privileges. Because a current user database link is used, the connection is established to scott's remote schema--not ford's remote schema. Note that scott must be a global user while ford does not have to be a global user.

Note:

If a connected user database link were used instead, the connection would be to ford's remote schema. For more information about invoker's-rights and privileges, see the PL/SQL User's Guide and Reference.

You can accomplish the same result by using a fixed user database link to scott's remote schema. With fixed user database links, however, security can be compromised because scott's username and password are available in readable format in the database.